1. Field of the Invention
The present invention relates to apparatus for transferring images. More particularly, the present invention relates to apparatus for carrying out multitransfer of images onto a transfer material supported on circulating transfer material bearing means.
2. Description of the Prior Art
A multitransfer apparatus is known in the art and used, for example, in a multicolor copying machine. In the known multicolor copying machine, images in different colors are sequentially formed on an image bearing member such as a photosensitive drum. The individual color images are then transferred onto a transfer material from the image bearing member by multiple transferring. In transferring, the transfer material is supported on transfer material bearing means having a dielectric surface in the form of sheet or mesh. Under the action of corona discharge, each of the color images is transferred onto the transfer material supported by said bearing means. After transferring, the transfer material is separated from the photosensitive drum and said transfer material bearing means carries the separated transfer material on its surface. At this step of separation, it is desirable that the transfer material subjected to the action of transfer corona be separated from the drum surface while keeping the transfer material in close contact with the surface of transfer material bearing means. However, in practice, the transfer material cannot completely be separated from the photosensitive drum at once after passing over the transfer position at which said transfer material bearing means and the photosensitive drum come into contact with each other for transferring. Even after the transfer material has already passed over the transfer position, the transfer material continues to be in close contact with the photosensitive drum some distance. Therefore, the transfer material gets in contact with said transfer material bearing means only when the transfer material has moved some distance together with the photosensitive drum after transferring. Even if the transfer material is separated from the photosensitive drum at once after transferring, the transfer material does not come into close contact with the surface of transfer material bearing means but moves floating in the space between the photosensitive drum and said transfer material bearing means. The transfer material gets in contact with the surface of said bearing means only when it has moved floating some distance after transferring. In either case, after transferring, the transfer material moves some distance in the state of floating in the air above the surface of said transfer material bearing means. In the known apparatus, therefore, some problems have been caused by such floating movement of the transfer material after transferring.
Firstly, the separation position at which the transfer material is separated from the surface of the photosensitive drum is not constant but variable which gives an adverse effect on the transferred image on the transfer material. Since the floating transfer material moves in unstable manner, some distortion is produced in the transferred images on the transfer material by offset etc. Such distorted images have difficulty in copy making in particular when it is desired to obtain images of high resolution.
Secondly, the floating movement of the transfer material leads to the problem of out-of-registration in multicolor image. When the transfer material is out of the surface of transfer material bearing means, the distance moved by the transfer material becomes longer than the normal passage along the surface of said transfer material bearing means. In case of the multi-transfer, the floating movement of transfer material is at the greatest degree for the first color. From the second color on, the degree of floating is reduced or there occurs no floating of the transfer material. Therefore, the first color image and other color images from the second color on transferred on the transfer material are not in registration but out of registration in color. Such error in registration will result in degraded reproduction of multicolor image.
Lastly, the floating movement of the transfer material constitutes the main cause for wrong conveyance of transfer material.
The mechanism of the above floating phenomenon of the transfer material has not been made fully clear yet. But, it may be said that the floating phenomenon occurs when the electrostatic attraction force of the transfer material bearing member acting on the transfer material is overcome by the attraction force of the photosensitive drum acting on the same transfer material. Also, it is believed that the floating phenomenon is partly attributable to the stiffness of the transfer material itself and to the curvature of the surface of said transfer material bearing means. The present invention is based on this understanding of the floating phenomenon.